Edgar Alan Poe, in his poem entitled "The Bells", enumerated many ways in which bells may be used to indicate a wide variety of conditions and events. Poe contemplated principally the wide variety of bells which were struck with a clapper. Currently, the generic term "bells" also includes a wide variety of electrically operated devices, one of the most ubiquitous of which is the ordindary houshold doorbell comprising a gong which is repetitively struck in response to the actuation of an electromagnet. With the advance in technology, bell tones have been amplified and gongs and strikers have been designed to produce a wide variety of tones and sounds. In addition, electronic techniques have been used to generate a wide variety of other audible alarm signals. Police and/or ambulance sirens, as used in many municipalities, are typical and offer various advantages in sound volume, ruggedness, economy and reliability.
Because of the generations of use of percussive bell signals and our familiarity with and acceptance of their sound, there are still many applications wherein it is considered desirable to use percussive bells. However, percussive bells and their associated electromechanical striker mechanisms have a tendency to be unreliable and/or require routine adjustment and/or maintenance. In addition, these traditional devices tend to be bulkier and more expensive than electronic sound generation. Accordingly, in order to provide traditional bell tones and electronic economy and reliability, efforts have been made to reproduce bell sounds electronically. For the most part, such devices have merely imitated bell sounds and have included a wrong mix of harmonics to simulate authentic sounds. Other techniques have required such extensive and elaborate circuitry as to render then uneconomic except in highly specialized applications.
Copending application Ser. No. 323,520 filed Nov. 19, 1981 by Harry Ferguson, is entitled Electric Simulation of Percussive Bell and which is assigned to the same assignee as the present invention discloses circuit means for simulating the sound of a percussive bell and employs a square wave generator and a sine wave generator for producing signals of different frequencies with the square wave signal filtered to remove selected harmonics. The remaining signal and the other signals are each modulated with different decaying exponential control signals and the resultant signals mixed to provide a signal suitable for audio amplification.
Other examples of prior art devices may be seen in the following patents:
U.S. Pat. No. 2,354,699 issued Aug. 1, 1944 to E. L. Owens is a pertinent patent in that it teaches generation of voltages of the more important frequencies of the desired tones and the blocking of an amplifier with a decaying signal characteristic of percussion type signals.
U.S. Pat. No. 3,325,578, issued June 13, 1967 to D. M. Park, teaches the use of two tuned circuits which produces frequencies which are not harmonically related. A triggering pulse source causes damped oscillations in the tuned circuits and exponentially decaying sound for simulating a cow bell.
U.S. Pat. No. 3,460,136, issued Aug. 5, 1969 to C. M. Jambazian, provides a device in which two signals of different frequencies are operated on to produce an output providing characteristics similar to the sound produced by birds and the like.
U.S. Pat. Nos. 3,218,636 and 3,742,492, issued Nov. 16, 1965 and June 26, 1973 to J. M. Bernstein et al. and D. F. Proctor, respectively, disclose techniques for producing sounds electronically and use piezoelectric devices.
U.S. Pat. No. 2,455,472, issued Dec. 7, 1948 to H. C. Curl et al. discloses a means for the selective generation of selected complex tones by frequency modulation to produce signals having a large number of frequency components.
U.S. Pat. Nos. 3,249,933 and 4,092,893, issued May 3, 1966 and June 6, 1978 to R. W. McKee and R. O. Beach, respectively, teach sound generation through amplifiers after striking a vibrating member.
U.S. Pat. No. 3,587,094, issued June 22, 1971 to R. Scott, teaches a generation of a variety of sounds through the use of random voltage generators, voltage controlled tone generators, pulsers, triggers, pulse shapers, keyers, audio generators, delay devices, amplifiers and loud speakers.
U.S. Pat. No. 4,180,808, issued Dec. 25, 1979 to J. P. Lebet et al. discloses another system using a piezoelectric transducer together with a means for controlling the applied potential.